System and method for selecting zero doppler transmission

ABSTRACT

An illustrated embodiment disclosed herein is a method including receiving, by an endpoint, a downlink (DL) PDU from of a plurality of sources, determining, by the endpoint and for each DL PDU, a frequency offset from a reference frequency, selecting, by the endpoint, one of the plurality of sources having a first frequency offset that satisfies an threshold, and sending, by the endpoint, an uplink (UL) PDU to the selected source.

BACKGROUND

The following description is provided to assist the understanding of thereader. None of the information provided or references cited is admittedto be prior art.

Doppler Multichannel Spread Spectrum (DMSS) is a new technology thataddresses the connectivity needs of the tremendously large Global AssetTracking and Monitoring Market. DMSS features global indoor penetrationand unprecedently low connectivity costs. Existing communication systems(including the cellular approaches of LTE-M and NB-IotT) will notprovide the global scale or the required price-points for commercialsuccess.

SUMMARY

An illustrated embodiment disclosed herein is a method includingreceiving, by an endpoint, a downlink (DL) PDU from of a plurality ofsources, determining, by the endpoint and for each DL PDU, a frequencyoffset from a reference frequency, selecting, by the endpoint, one ofthe plurality of sources having a first frequency offset that satisfiesan threshold, and sending, by the endpoint, an uplink (UL) PDU to theselected source.

Another illustrated embodiment disclosed herein is a system including aplurality of sources and an endpoint, wherein the endpoint havingprogrammed instructions that when executed cause an endpoint processorat the end point to receive a downlink (DL) PDU from of the plurality ofsources, determine, for each DL PDU, a frequency offset from a referencefrequency, select one of the plurality of sources having a firstfrequency offset that satisfies an threshold, and send an uplink (UL)PDU to the selected source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example block diagram of a communication system, inaccordance with some embodiments.

FIG. 2 is an example block diagram of an endpoint, in accordance withsome embodiments.

FIG. 3 is an example downlink frame, in accordance with someembodiments.

FIG. 4 is an example plot of Rayleigh fading, in accordance with someembodiments.

FIG. 5 is an example method for employing frequency diversity, inaccordance with some embodiments.

FIG. 6 is an example method for determining a target for an uplinktransmission, in accordance with some embodiments.

FIG. 7 is an example method for performing a power control scheme, inaccordance with some embodiments.

FIG. 8 is an example method for recovering a payload encoded by forwarderror correction, in accordance with some embodiments.

FIG. 9 is an example method for implementing a downlink schedule, inaccordance with some embodiments.

FIG. 10 is an example method for processing satellite detection, inaccordance with some embodiments.

FIG. 11 is an example timing diagram for Doppler frequency offset, inaccordance with some embodiments.

FIG. 12 is an example method for second derivative compensation, inaccordance with some embodiments.

DETAILED DESCRIPTION

One aspect of the disclosure is a frequency diversity scheme thatestimates the signal gain or loss of a communication link. In someembodiments of the disclosure, the systems and methods described hereincan select a channel for which the communication link has a signal gaingreater than a signal gain of any other channel. In some embodiments,the systems and methods described herein can increase a link budget inthe presence of Rayleigh fading through frequency diversity.

Another aspect of the disclosure is a system and method for determininga target for sending an uplink transmission. In some embodiments, thesystem and method can determine, for each downlink transmission, whetherthe transmission is sent by a tower or satellite. In some embodiments,the system and method can select a target for uplink transmission basedon Doppler characteristics of the corresponding downlink transmission.In some embodiments, the system and method can prioritize communicationwith towers, thus reducing latency, power consumption, and operationalcosts.

Another aspect of the disclosure is a system and method for performing apower control scheme. In some embodiments, the satellite selects anoptimal spreading factor (SF) and the endpoint demodulates all the SFsto determine the signal at the SF computed by the satellite. In someembodiments, the power control scheme optimizes downlink (DL) capacity.

Another aspect of the disclosure is a system and method for recovering apayload encoded by forward error correction. In some embodiments,encoding is done at the endpoint and extra parity PDUs are sent to oneor more satellites. In some embodiments, a gateway collects the PDUstagged with MAC ID and SDU sequence number and when enough PDUs arereceived the gateway can decode to an original payload. One applicationis in a handover during transmission of a datagram/SDU/payload.

Another aspect of the disclosure is a system and method for implementinga downlink schedule. In some embodiments, a satellite can select sendingphysical data units (PDUs) with smallest SFs. Once the satellite hasadded PDUs of the smallest SF to a slot, the satellite can iterativelyincrease the SF and add additional PDUs until the slot is full. In someembodiments, such a schedule allows for optimization of DL capacitybecause the satellite can pack into each frequency slot as efficientlyas possible.

Another aspect of the disclosure is a system and method for satellitedetection. In some embodiments, an endpoint correlates frames withhypotheses. In some embodiments, the endpoint determines a subset ofhypotheses that have strongest correlations for the first frame and thesecond frame. In some embodiments, the endpoint selects a hypothesisthat is in both subsets and has a highest received signal in the secondframe among all the hypotheses in both of the first and second subsets.The proposed aspect has a lower detection time than simple thresholding.

Another aspect of the disclosure is a system and method for secondderivative compensation. In some embodiments, an endpoint determines asecond derivative of a frequency offset based on the measured frequencyoffset and its first derivative. In some embodiments, the endpointpredicts the frequency offset and its first derivative at a later timebased on the calculated second derivative. By more accurately predictingthe frequency offset characteristics, the system (e.g., the endpoint ora corresponding satellite) can reduce the search space for the uplink(UL) signal. As a result, the system saves at least one of power, gatecount, or field-programmable gate array (FPGA) size/silicon area.

Referring now to FIG. 1, an example communication system 100 is shown.The communication system 100 can include one or more endpoints 102, oneor more satellites 104, a ground station 106, a communication gateway108, and one or more applications 110(1)-110(N) (generally referred toas “application 110”). The one or more endpoints can receive andtransmit one or more Service Data Units (“SDUs”). Each SDU can includeone or more physical layer Physical Data Units (“PDUs”). Each PDU caninclude a data stream, data, symbols, and/or bits. In some embodiments,each PDU includes 192 bits totally independent of a spreading factor.Each PDU can include a dedicated (e.g., unicast) data or a broadcastdata. Each PDU can have a corresponding spreading factor (“SF”).

The one or more endpoints 102 is coupled to the one or more satellites104 with one or more device links. A device link can be established whenan endpoint 102 acquires a satellite 104 downlink signal. The one ormore satellites 104 can transmit SDUs and PDUs to, or receive SDUs andPDUs from, the one or more endpoints 102. In some embodiments, the oneor more satellites 104 include one or more (e.g., terrestrial) towers.In some embodiments, the one or more towers are separate from the one ormore satellites 104.

The one or more satellites 104 is coupled to the ground station 106 withone or more backhaul links. The satellite can process an SDU down to thePDU layer and send the one or more PDUs to the gateway 108 via theground station 106. The satellite can receive PDUs from the gateway 108via the ground station 106. The ground station 106 is coupled to thecommunication gateway 108. The gateway 108 can decode the one or morePDUs to one or more user payloads and can encode the one or more userpayloads to the one or more PDUs. The communication gateway 108 iscoupled to the one or more applications 110 with one or more data pathinterfaces.

Referring now to FIG. 2, an example block diagram of an endpoint 102 isshown. The endpoint 102 can include a downlink (“DL”) path 202 and anuplink (“UL”) path 204. The DL path 202 of the endpoint 102 processesPDUs received from the satellite 104 in units called frames. The DL pathcan include an automatic frequency control (“AFC”) 208, a detector 216,a finger selector 220, a despreader 224, a demodulator 228, and adecoder 230. In some embodiments, each of the components of the endpoint102 may include one or more processing devices (e.g., a digitalprocessor, an analog processor, a digital circuit designed to processinformation, an analog circuit designed to process information, a statemachine, and/or other mechanisms for electronically processinginformation). The one or more processing devices may include one or moredevices executing operations in response to instructions storedelectronically on an electronic storage medium.

In some embodiments, the DL path 202 performs timing and frequencyacquisition of a DL PDU on a broadcast channel such as a systeminformation block (“SIB”) channel and/or a constellation (“CONST”)channel. The AFC 208 can receive the broadcast PDU from an analog todigital converter (“ADC”). The AFC 208 explores different Dopplerfrequency hypotheses (e.g., frequencies, phases, timing). In someembodiments, a Doppler window +/−60 kHz is explored. In someembodiments, each of the hypotheses have a resolution of 244 Hz. In someembodiments, the AFC 208 derotates the broadcast PDU by at least one ofa frequency or phase. In some embodiments, each Doppler hypothesis is afrequency version or portion of the broadcast PDU. In some embodiments,the Doppler hypothesis compensates for a Doppler offset frequency or asum of the Doppler offset frequency and a TCXO error frequency offset.The AFC 208 can include a lookup table (“LUT”) and an accumulator havingan accumulation rate corresponding to the Doppler hypothesis. A columnin the lookup table can be selected for each sample based on an outputof the accumulator.

The detector 216 detects (e.g., accumulates, measures, or otherwisedetermines) a signal (e.g., metric, level, power, energy, receivedsignal, received signal strength, signal-to-noise ratio, etc.) eachDoppler hypothesis of the broadcast PDU. In some embodiments, thedetector 216 correlates each Doppler hypothesis with a known pseudonoise (PN) sequence to generate a correlated signal and detects eachcorrelated signal. The correlation can be coherent, non-coherent, or acombination thereof. The detector 216 can include a matched filter. Thefinger selector 220 can rank the Doppler hypotheses based on thedetected signal of the Doppler hypothesis. In some embodiments, thefinger selector 220 selects N broadcast PDU hypotheses having thehighest detected signal. The finger selector 220 can identify (e.g.,select, determine, apply, etc.) different timing (e.g., chip timing)hypotheses for each Doppler hypothesis.

The despreader 224 can despread each of the timing hypotheses of each ofthe Doppler hypotheses of the DL PDU selected by the finger selector220. The despreader 224 can despread the M×N selected hypotheses of thebroadcast PDU from the chip rate to the symbol rate, wherein M is anumber of timing hypotheses and N is a number of Doppler hypotheses. Thedespreader 224 can despread the M×N selected hypotheses by multiplyingthe hypothesis by a PN sequence and summing the multiplications. The PNsequence may be a Gold code or some other code. The PN sequence may beknown a priori. The PN sequence may have a spreading factor (“SF”). TheSF may be known a priori. The despreader 224 can include a multiplierhaving the PN sequence as the second input. The despreader 224 caninclude an accumulator coupled to the output of the multiplier, and adecision block coupled to the output of the accumulator.

The demodulator 228 can demodulate each of the M×N selected hypotheses.The modulation type being demodulated can include differential phaseshift keying (“D-PSK”).

In some embodiments, the DL path 202 demodulates DL PDUs (e.g., havingdedicated data). The DL path 202 can have similar operations fordemodulating a dedicated DL PDU as for acquiring a broadcast DL PDUexcept for differences described herein. In some embodiments, theDoppler frequency offset and the chip timing of the DL PDU are derivedfrom detecting and/or demodulating the broadcast PDU. The AFC 208 canderotate one dedicated DL PDU reliably. In some embodiments, thedetector 216 and finger selector 220 can be bypassed. In someembodiments, the detector 216 can be used to detect auxiliary signals(e.g., sequences) such as a swept synchronization (“sync”). Thedetection of the swept sync can be used for frequency diversity, whichis described herein. The swept syncs can be received as a part of acommon frame that includes the dedicated DL PDUs. The despreader 224 candespread the DL PDU with one or more SFs.

The decoder 230 (e.g., Reed-Solomon decoder, Viterbi decoder) receives(e.g., the selected hypothesis of the broadcast (or dedicated) DL PDU(e.g., n encoded symbols, including k information symbols and n-k paritycheck symbols) from the demodulator 228. In some embodiments, thedecoder 230 decodes, forward error corrects, or otherwise recovers the kinformation symbols from at least a portion of the n encoded symbols. Insome embodiments, the decoder 230 uses “m” previous broadcast PDUs todecode the current broadcast PDU. In some embodiments, the decoderincludes a demultiplexer, a plurality of parallel forward errorcorrection (FEC) decoders coupled to the demultiplexer, and amultiplexer.

The decoder 230 can deinterleave the broadcast (or dedicated) DL PDU.The decoder 230 can check the cyclical redundancy check (“CRC”). Thedecoder 230 can identify a DL PDU corresponding to the correct Dopplerhypothesis and the correct chip timing hypothesis. The identifiedbroadcast PDU can be identified as having a correct CRC. The decoder 230can send the identified broadcast PDU to the CPU 232.

The UL path can include an encoder 234, a modulator 236, a spreader 240,an upsampler 244, and an AFC 248. In some embodiments, the UL path 202transmits UL PDUs (e.g., broadcast data to multiple satellites ordedicated data to one satellite). The encoder 234 can encode the UL PDUusing FEC codes, thus increasing the UL PDU from k information symbolsto n encoded symbols. The encoder 234 can encode and interleave the ULPDU. The modulator 236 can modulate the UL PDU with a modulation schemesuch as D-PSK. The spreader 240 can spread the UL PDU from a symbol rateto a chip rate using a PN sequence. The upsampler 244 can upsample theUL PDU. The AFC 248 can rotate the UL PDU with a frequency offset.

In some embodiments, the satellite 104 can include components similar tothe endpoint 102, such as the components with respect to FIG. 2.

Referring now to FIG. 3, an example frame 300 is shown. The frame 300includes uplink (UL) frame 310 and downlink (DL) frame 320. Each of ULframe 310 and DL frame 320 includes slots 302(1)-(N) (generally referredto as a slot 302). In some embodiments, each slot is assigned to,allocated to, transmitted at, or otherwise corresponds to a resource(e.g., channel) in time, frequency, or both. For example, slot 302(2)can correspond to channel 39 and slot 302(3) can correspond to channel34. Each slot 302 can include one or more PDUs 306 and a slottedsynchronization (sync) 308 (e.g., slotted sync signal). Thus, forexample, slot 302(1) includes one or more PDUs 306(1) and a slotted syncsignal 308(1). Each of the PDUs 306 can include data, a Viterbi tail, aCRC, a MAC ID, a MAC header, and/or data. Broadcast channels can be usedto transmit broadcast PDUs (e.g., CONST or SIB) and dedicated channelscan be used to transmit dedicated PDUs (e.g., DL F0, DL F1, etc.) Insome embodiments, each of the UL frame 310 and the DL frame 320 includesswept syncs 304(1)-304(M) (collectively referred to herein as sweptsyncs 304 or swept sync signals 304). In some embodiments, only the DLframe 320 includes swept syncs 304. The swept syncs 304 can be used todetermine a channel for the endpoint 102 to transmit UL data on.

Referring now to FIG. 4, an example plot 400 of Rayleigh fading isshown. Rayleigh fading typically occurs when communication signals arewirelessly transmitted in an indoor environment. The x-axis of the plot400 represents the frequency of transmission between the endpoint 102and the satellite 104. The y-axis of the plot 400 represents the signaltransmission gain. Each of the channels in a frequency band are plottedat their corresponding transmission frequencies and corresponding signalgains. As a result of the Rayleigh fading, some channels, such aschannel 402, may experience signal loss compared to a case of noRayleigh fading. Other channels, such as channel 404, may experiencesignal gain compared to a case of no Rayleigh fading.

Referring now to FIG. 5, an example method 500 for employing frequencydiversity is shown, in accordance with some embodiments. The method 500may be implemented using, or performed by, one or more of the componentsof the endpoint 102, which is detailed herein with respect to FIG. 1 andFIG. 2. Additional, fewer, or different operations may be performed inthe method 500 depending on the embodiment.

At block 502, the endpoint 102 receives the multiple swept syncs 304transmitted by the satellite 104 on a DL frame such as the DL frame 320of FIG. 3. Each of the swept syncs 304 can be transmitted on a differentchannel (e.g., at a different frequency). In some embodiments, there isone swept sync 304 for each channel. In some embodiments, the sweptsyncs 304 are at the end of the DL frame.

At block 504, the endpoint 102 measures (e.g., detects) the receivedpower of the multiple swept syncs 304. In some embodiments, the power isthe average energy during a predetermined time interval. In someembodiments, the power is the peak energy during the predetermined timeinterval. The AFC 208 may derotate each of the swept syncs 304appropriately (e.g., to a baseband frequency). The detector 216 maydetect energy in each of the swept syncs 304 (e.g., at the basebandfrequency). In some embodiments, the AFC 208 derotates the swept sync304 signals one at a time, and the detector 216 detects each of theswept sync 304 signals one at a time. In some embodiments, the sweptsync 304 signals may be stored in a buffer prior to being sent to theAFC 208.

At block 506, the endpoint 102 determines (e.g., identifies, selects,etc.) a first swept sync based on the received power. In someembodiments, the endpoint 102 determines a first swept sync having ahighest received power (e.g., signal strength, power, or other metric)among all of the swept syncs 304.

At block 508, the endpoint 102 interpolates (e.g., maps, links, etc.)from the swept sync to a first channel for sending an UL PDU. In someembodiments, the endpoint 102 interpolates from the plurality ofchannels used for swept syncs 304 to a plurality of channels used for ULPDUs (e.g., including the first channel). For example, there are 9 sweptsync channels and 16 UL PDU channels. Thus, swept sync channels may notalign with the UL PDU channels. In some embodiments, the first channelthat is interpolated from the swept sync channel overlaps with the sweptsync channel. In some embodiments, the first channel that isinterpolated from the swept sync channel does not overlap with the sweptsync channel.

At block 510, the endpoint 102 sends an UL PDU or signal on the firstchannel. In some embodiments, the satellite 104 receives the UL PDU fromthe endpoint 102 and other UL PDUs from other endpoints. In someembodiments, the UL PDUs from the other endpoints are on other channels.In some embodiments, the UL PDU may include a MAC ID or an SDU sequencenumber. The satellite 104 may determine the source endpoint of eachsignal. For example, the satellite 104 may extract the MAC ID or the SDUsequence number from the UL PDU and map the MAC ID or the SDU sequencenumber to the endpoint 102.

At block 512, the endpoint 102 receives a DL PDU 306 or signal on asecond channel based on the first channel. In some embodiments, thesecond channel is closer to the first channel than any other of the DLchannels. In some embodiments, the second channel is overlapping infrequency with the first channel or separate in (e.g., non-overlappingwith) frequency from the first channel. In some embodiments, thesatellite 104 selects the second channel. In some embodiments, theendpoint 102 and the satellite 104 leverage (e.g., exploit) channelreciprocity. In some embodiments, the second channel may have a same orsubstantially similar Rayleigh fading signal gain as the first channel.

In some embodiments, blocks 502-512 may be repeated such that the firstchannel is continuously updated. For example, the blocks 502-510 mayoccur during frame N and the block 512 may occur during frame N+1. Insome embodiments, each of the blocks may occur during every frame. Insome embodiments, the endpoint 102 receives a second plurality of sweptsyncs in a same DL frame (e.g., frame N+1) as the DL PDU. In someembodiments, the endpoint 102 measures a second received power for eachof the second plurality of swept syncs. In some embodiments, theendpoint 102 determines a third channel corresponding to a second sweptsync having the highest measured power in frame N+1. In someembodiments, the second swept sync having the highest measured power inframe N+1 is on a different channel than the swept sync having thehighest measured power in the DL frame (e.g., frame N).

Referring now to FIG. 6, an example method 600 for determining a targetfor an uplink transmission is shown, in accordance with someembodiments. The method 600 may be implemented using, or performed by,one or more of the components of the endpoint 102, which is detailedherein with respect to FIG. 1 and FIG. 2. Additional, fewer, ordifferent operations may be performed in the method 600 depending on theembodiment.

At block 602, the endpoint 102 can receive frames, such as the frame300, from multiple sources. Each frame includes one or more DL signals.In some embodiments, the one or more DL signals include at least one DLPDU, such as PDU 306(N), and one swept sync, such as swept sync 304(M).The multiple sources may include one or more satellites 104 and one ormore towers.

At block 604, the endpoint 102 can determine, for each DL PDU from eachof the multiple sources, a frequency shift or offset from a referencefrequency. The endpoint 102 can know (e.g., determine) a priori what theexpected frequency of a DL PDU is. The endpoint 102 (e.g., thecomponents of the endpoint 102) uses the swept sync as a proxy for theDL PDU. The AFC 208 can derotate the swept sync by the expectedfrequency. The detector 216 can measure the power or energy in multiplefrequency bands. In some embodiments, the detector 216 can measureenergy within a predetermined frequency range such as −60 kHz to 60 kHz,and the range can be partitioned into the multiple frequency bands. Thedetector 216 can determine that the frequency shift is the centerfrequency of the frequency band having the most power or energy. In someembodiments, the demodulator 228 may demodulate the energy and check theCRC to determine whether the energy is that of a swept sync.

At block 606, the endpoint 102 can select one of the multiple sources tobe the target for transmitting a UL signal (e.g., a UL PDU and/oracknowledge of the DL signal). The endpoint 102 can select one of thesources whose swept sync satisfies a frequency shift threshold. Forexample, the endpoint 102 can select one of the sources whose swept synchas a frequency offset between −1 kHz and 1 kHz. In some embodiments,satisfying the frequency shift threshold may indicate that the source isa tower. In some embodiments, the endpoint 102 may determine that morethan one of the sources satisfies the frequency shift threshold. In someembodiments, the endpoint 102 may use a secondary metric to select asource from the group of sources that satisfies the frequency shiftthreshold. For example, the secondary metric may be the received power.In some embodiments, the endpoint 102 can select one of the sourceswhose swept sync satisfies a received power threshold.

At block 608, the endpoint 102 may send the UL signal to the source thatis selected to be the target. In some embodiments, the endpoint 102leverages channel reciprocity by selecting an UL channel based on (e.g.,closest to, most closely mapped) the channel at which the DL frame wasreceived. In some embodiments, the endpoint 102 may apply the samefrequency shift to the UL signal that was identified in the swept syncreceived from the selected source.

Referring now to FIG. 7, an example method 700 for performing a powercontrol scheme is shown, in accordance with some embodiments. The method700 may be implemented using, or performed by, one or more of theendpoint 102 and the satellite 104, which are detailed herein withrespect to FIG. 1 and FIG. 2. Additional, fewer, or different operationsmay be performed in the method 700 depending on the embodiment.

At block 702, the satellite 104 measures, for each uplink channel, arise-over-thermal (e.g., at the satellite 104 location). Therise-over-thermal indicates an amount of noise and interference fromendpoints and external sources. In some embodiments, therise-over-thermal varies across the uplink channels. Therise-over-thermal measurement can be an absolute measurement. In someembodiments, a thermal noise floor is known a priori. In someembodiments, the satellite 104 computes a rise-over-thermal delta as adifference of the rise-over-thermal and the thermal noise floor.

At block 704, the satellite 104 reduces a DL Tx power for a portion(e.g., swept syncs 304) of each of the DL frames, such as the DL frame320 with respect to FIG. 3, to be transmitted to one or more endpointssuch as endpoint 102. The remaining portion(s) (e.g., DL PDUs, slottedsyncs, etc.) of the DL frame are referred to as the full power portionof the DL frame. In some embodiments, the reduction in the Tx power ofthe reduced power portion of the DL frame is based on therise-over-thermal measurement of the uplink channel corresponding to(e.g., nearest the downlink channel of) the reduced power portion of theDL frame. For example, if the rise-over-thermal is 3 dB above the noisefloor, then the satellite reduces the Tx power by 3 dB. In someembodiments, the satellite 104 reduces the Tx power by backing off Txgain. In some embodiments, the satellite 104 broadcasts the first DLframe to all endpoints in range and the endpoint 102 receives the firstDL frame.

At block 706, the endpoint 102 measures a received signal strengthindicator (RSSI) of the reduced power portion of the DL frame (e.g.,after the endpoint 102 receives the DL frame from the satellite 104). Insome embodiments, the measured RSSI is lower due to the satellite 104backing off the DL Tx power.

At block 708, the endpoint 102 selects a UL SF and a UL Tx backoff. Insome embodiments, the endpoint transmits a first UL frame, such as theUL frame 310 with respect to FIG. 3, at a target UL Tx power by usingthe selected UL SF and UL Tx backoff. In some embodiments, the endpoint102 selects the UL SF and the Tx backoff based on the measured RSSI. Insome embodiments, the endpoint 102 initially sets the SF to a maximumpermissible SF (e.g. SF 1024) and the Tx backoff to 0 dB.

In some embodiments, the endpoint 102 determines a first predeterminedthreshold and a second predetermined threshold based on the RSSI. Insome embodiments, the endpoint 102 reduces the UL SF until the firstpredetermined threshold is satisfied. In some embodiments, the firstpredetermined threshold is that a predicted UL Tx power is within 3 dBof the target UL Tx power. In some embodiments, the SF is in steps thatresults in 3 dB changes in UL Tx power. In some embodiments, theendpoint 102, responsive to satisfying the first predeterminedthreshold, increases the Tx backoff until the second predeterminedthreshold is satisfied. In some embodiments, Tx backoff is in steps of0.1 dB. In some embodiments, the second predetermined threshold is thata predicted UL Tx power is equal to, substantially equal to, or within0.1 dB, the target UL Tx power.

At block 710, the endpoint 102 measures a rise-over-thermal of thereduced power portion of the DL frame (e.g., at the endpoint 102location). In some embodiments, the endpoint 102 measures therise-over-thermal for a designated channel at which data (e.g., PDUs) iscommunicated between the endpoint 102 and the satellite 104. In someembodiments, the rise-over-thermal at the endpoint 102 location isdifferent from (e.g., greater than or less than) the rise-over-thermalat the satellite 104 location. In some embodiments, therise-over-thermal at the endpoint 102 location is written to or storedin a PDU 306 of the first UL frame.

At block 712, the endpoint 102 informs (e.g., transmits to, etc.) thesatellite 104 of at least one of the measured rise-over-thermal at theendpoint 102 location or the UL SF. In some embodiments, at least one ofthe endpoint 102's rise-over-thermal or the UL SF is written to a PDU306 of the first UL frame. In some embodiments, the endpoint 102transmits the unicast, first UL frame and the satellite 104 receives theunicast, first UL frame.

At block 714, the satellite 104 selects, calculates, or otherwisedetermines a DL SF for one or more DL PDUs 306 of a second DL frame. Insome embodiments, the satellite 104 selects the DL SF based on at leastone of the satellite 104's measured rise-over-thermal (e.g., measured in702), the endpoint 102's measured rise-over-thermal (e.g., measured in710), or the UL SF. In some embodiments, the satellite 104 computes afirst delta between the satellite 104's measured rise-over-thermal andthe endpoint 102's measured rise-over-thermal. In some embodiments, thefirst delta is computed as a first Tx power of the reduced power portionof the DL frame and a second Tx power of the UL frame. For example, thefirst delta can be converted to a first SF or SF delta (e.g., a 3 dBpower delta is the same as 2 x SF). In some embodiments, the satellite104 selects a DL SF is equal to a function (e.g., sum, difference,product, quotient, etc.) of the first SF delta and the UL SF. In someembodiments, the satellite 104 transmits a unicast, second DL frame andthe endpoint 102 receives the unicast, second DL frame. In someembodiments, the satellite 104 selects a Tx backoff for transmitting theDL PDUs 306 of the second DL frame.

At block 716, the endpoint 102 simultaneously demodulates the second DLframe at multiple spreading factors. In some embodiments, the endpoint102 demodulates each of the DL PUDs 306 in the second DL frame using acode (e.g., one of the Gold Codes). Each code corresponds to one of themultiple SFs. In some embodiments, the endpoint 102 successfullydemodulates a DL PDU 306 that is demodulated with a first codecorresponding to a first SF. In some embodiments, the endpoint 102identifies correct CRC bits in a demodulated DL PDU 306 that isdemodulated with a first code corresponding to a first SF. In someembodiments, the endpoint 102 determines the DL SF to be the first SF.

Referring now to FIG. 8, an example method 800 for recovering a payloadencoded by forward error correction is shown. The method 800 may beimplemented using, or performed by, one or more of the endpoint 102, themultiple satellites 104, or the gateway 108, which are detailed hereinwith respect to FIG. 1 and FIG. 2. Additional, fewer, or differentoperations may be performed in the method 800 depending on theembodiment.

At block 802, the endpoint 102 encodes a payload (e.g., payload,datagram, SDU, etc.) to multiple (e.g., N) PDUs. The endpoint 102 usesforward error correcting (FEC), such as FEC codes, erasure codes,Reed-Solomon codes, to encode the payload. The multiple PDUs includeoriginal data of the payload and redundant data, e.g., parity bitsand/or symbols. In some embodiments, the multiple PDUs includes at leastone parity PDU. In some embodiments, the at least one parity PDUincludes a portion of the redundant data. In some embodiments, the atleast one parity PDU includes a portion of the payload. In someembodiments, each of the multiple PDUs are transmitted on a unicast(e.g., dedicated) channel.

At block 804, the endpoint 102 tags, assigns, or otherwise associateseach of the multiple PDUs with a same (e.g., one or more) identifier(s).In some embodiments, the identifier includes a MAC ID and/or an SDUsequence number. At block 806, the endpoint 102 sends the multiple PDUs,including a first PDU and a second PDU, to multiple satellites 104(e.g., the endpoint 102 sends the first PDU to a first satellite 104 andthe second PDU to a second satellite 104). In some embodiments, prior tosending the multiple PDUs, the endpoint 102 modulates and/or spreads themultiple PDUs.

At block 808, at least one of the multiple satellites 104 (e.g., thefirst satellite 104) forwards one of the multiple PDUs (e.g., the firstPDU) to the gateway 108. In some embodiments, at least one of themultiple satellites 104 despreads and/or demodulates the one of themultiple PDUs. At block 810, the gateway 108 receives (e.g., via theground station 106) at least one of the multiple PDUs.

At block 812, the gateway 108 determines whether the identifier matchesa local identifier. In some embodiments, the gateway 108 compares eachof multiple local identifiers to the identifier until finding a match.In some embodiments, the multiple local identifiers are located orstored in a data structure, memory, solid state drive, or other storagemedium. In some embodiments, in response to not finding a match, thegateway 108 discards the PDU. In some embodiments, responsive todetermining that the gateway 108 determines that the identifier matchesthe local identifier, the method 800 proceeds to block 814.

At block 814, the gateway 108 determines whether a number of PDUs (e.g.,received and tagged with identifiers matching the local identifier)satisfies (e.g., is greater than) a predetermined threshold number ofPDUs. Responsive to the number of PDUs not being greater than thepredetermined threshold, the method 800 returns to block 810 (e.g.,receives one or more additional PDUs from the multiple satellites 104).Responsive to the number of PDUs satisfying the predetermined threshold,the method 800 proceeds to block 816.

At block 816, the gateway 108 decodes, reconstructs, or otherwiserecovers the payload from the number of PDUs. In some embodiments, thegateway decodes the payload from a first number of PDUs that is lessthan a second number of PDUs that the payload was encoded to (e.g., atblock 802).

Referring now to FIG. 9, an example method 900 for implementing adownlink schedule is shown. The method 900 may be implemented using, orperformed by, a satellite 104, which is detailed herein with respect toFIG. 1 and FIG. 2. Additional, fewer, or different operations may beperformed in the method 900 depending on the embodiment.

At block 902, the satellite 104 maintains a plurality of PDUs 306 (e.g.,DL PDUs 306), each PDU 306 having a corresponding spreading factor (SF).In some embodiments, each PDU 306 corresponds to an endpoint 102. Morethan one PDU 306 can correspond to a same endpoint. The satellite 104may separate (e.g., group, partition, or otherwise arrange) the PDUsinto groups based on which endpoint 102 each PDU 306 corresponds to. Insome embodiments, the satellite 104 assigns, allocates, or otherwisecorresponds each endpoint 102 to a slot 302 in a next DL frame (e.g.,the DL frame 320). In some embodiments, the satellite 104 determineswhich of the PDUs 306 to transmit in the slot 302 for the correspondingendpoint 102. The PDUs 306 which are not transmitted in the slot 302 canbe transmitted in a second slot 302 in a later DL frame.

In some embodiments, the satellite 104 calculates the SF to use for eachDL PDU 306. The calculation of a PDU's 306 SF may be based on a receivedpower level of a corresponding UL PDU. In some embodiments, thesatellite spreads each DL PDU 306 using its corresponding SF. In someembodiments, the satellite 104 separates the PDUs 306 into subsets basedon what endpoint 102 each of the PDUs 306 are destined (e.g., targeted,assigned) for. In some embodiments, the satellite 104 ranks or sorts theplurality of DL PDUs 306 (e.g., each of the subsets of DL PDUs 306) fromlowest SF to highest SF. The satellite 104 may store the PDUs and theranking of the PDUs in a memory or storage associated with the satellite104 or other component in the communication system 100. In someembodiments, each different SF corresponds to a different (e.g.,additional) overhead (e.g., overhead bits) in the PDUs corresponding tothat SF.

At block 904, the satellite 104 selects a lowest SF (e.g., a SF is lowerthan SFs of all other PDUs maintained by the satellite 104 and destinedfor a specific endpoint 102, a minimum SF, etc.). At block 906, thesatellite 104 adds, assigns, packs, picks, or otherwise corresponds aPDU 306 having the currently selected SF to the slot 302. Thus, in thefirst iteration, the currently selected SF is the lowest SF. In someembodiments, multiple PDUs 306 have the selected SF. In someembodiments, the satellite 104 randomly picks (e.g., selects) one of thePDUs 306 having the selected SF.

At block 908, the satellite 104 determines whether the availablecapacity of the slot 302 is less than a predetermined threshold. Thepredetermined threshold may be a fixed number of bits and/or a number ofbits sufficient to add a next PDU 306 to the slot 302. For example, thesatellite 104 determines whether the slot 302 is full. In anotherexample, the predetermined threshold is a fixed percentage between 0 and100 or a fixed amount.

If the satellite 104 determines that the available capacity of the slot302 is not less (e.g., greater) than the predetermined threshold, thenat block 910, the satellite 104 determines whether there are more PDUs306 (e.g., corresponding to the endpoint 102) having the selected SF toadd to the slot 302. If there are more PDUs (e.g., available PDUs)having the selected SF to add to the slot 302, then the method returnsto block 906.

If there are no more PDUs 306 having the selected SF to add to the slot302, or, in some embodiments, if there are more PDUs 306 having theselected SF that have not been scheduled, then, at block 912, thesatellite 104 determines whether there are PDUs 306 associated with anext SF (e.g., a greater SF, a SF that is greater than an SF of a mostrecently added PDU but lower than SFs of other remaining PDUs, etc.). Insome embodiments, the PDUs 306 and the corresponding SFs are stored(e.g., saved, written to, kept) in a lookup table (LUT). In someembodiments, the LUT is sorted in order of SF. In some embodiments,finding a next SF includes incrementing an index, reading an SF value,and repeating until the SF value is greater than a previous SF value orthe index is greater than a size of the number of PDUs.

If there is a next SF, then, at block 914, the satellite 104 selects thenext SF. After selecting the next SF, the method returns to block 906.If there is not a next SF, or if the satellite 104 determines (at block908) that the available capacity of the slot 302 is less than the secondpredetermined threshold, then, at block 916, the satellite 104 sends theslot 302 to the endpoint 102.

In some embodiments, the satellite 104 updates (e.g., re-calculates) theSFs of the PDUs 306 that are not added to the slot 302. In someembodiments, the update occurs prior to determining what PDUs 306 to addto a second slot 302. In some embodiments, the update is based onreceiving updated receive power levels of the corresponding UL PDUs. Insome embodiments, the received power level of the corresponding UL PDUshas increased since the transmission of the slot 302 (e.g., because of aless noisy or attenuated communication path between the satellite 104and an endpoint 102 corresponding to the slot 302), thereby resulting inan updated SF that is lower than the original SF.

Referring now to FIG. 10, an example method 1000 for satellite detectionis shown. The method 1000 may be implemented using, or performed by, theendpoint 102 and/or one or more of the components of the endpoint 102(e.g., the AFC derotator 208, the detector 216, the finger selector220), which are detailed herein with respect to FIG. 1 and FIG. 2.Additional, fewer, or different operations may be performed in themethod 1000 depending on the embodiment.

At block 1002, the endpoint 102 correlates a first frame, such as the DLframe 320, with multiple, different hypotheses and correlates a secondframe, such as the DL frame 320, with the multiple, different hypotheses(e.g., the endpoint 102 explores different hypotheses of the first frameand the second frame). The first and second frames are received by theendpoint 102 and from a satellite 104. In some embodiments, correlatingthe first frame includes correlating a first frame component (e.g.,swept sync, slotted sync, or PDU in the first frame) and correlating thesecond frame includes correlating a second frame component (e.g., sweptsync, slotted sync, or PDU in the second frame). Examples of swept sync,slotted sync, and PDU are swept sync 304(M), slotted sync 308(N), andPDU 306(N), respectively. In some embodiments, correlating the firstframe component and the second frame component includes at least one ofshifting or derotating the frequency (e.g., the AFC) of the first framecomponent and the second frame component, shifting (e.g., offsetting, orotherwise adjusting) the chip timing of the first frame component andthe second frame component, or shifting or derotating the phase of thefirst frame component and the second frame component by at least one ofa frequency, chip timing, or phase of a hypothesis. Each hypothesis mayinclude, or be associated with, at least one of a different frequencyoffset, different chip timing, or different phase.

At block 1004, the endpoint 102 detects (e.g., measures or otherwisedetermines), for each correlation of the first frame with a hypothesis,a first metric and, for each correlation of the second frame, a secondmetric (collectively, “a metric”). In some embodiments, the metric is anenergy metric such as received signal strength, signal-to-noise ratio,etc. In some embodiments, the endpoint 102 detects the metric of the DLPDU, the swept sync, or the slotted sync.

At block 1006, the endpoint 102 identifies a first subset of hypothesesbased on the first metric and a second subset of hypotheses based on thesecond metric. In some embodiments, the endpoint 102 identifies a firstsubset of hypotheses having a highest first metric and a second subsetof hypotheses having a highest second metric (e.g., or otherwisesatisfies a first predetermined or dynamic threshold). Thus, in someembodiments, each hypothesis in the first subset has a higher firstmetric than a first metric of any hypothesis not in the first subset.For example, if hypothesis A has an RSSI of −100 dBm, hypothesis B hasan RSSI of −110 dBm, and hypothesis C has an RSSI of −120 dBm, and thetop hypotheses with the two highest first metric are identified as thefirst subset, then the endpoint 102 identifies hypotheses A and B as thefirst subset. Similarly, each hypothesis in the second subset has ahigher second metric than a second metric of any hypothesis not in thesecond subset. In some embodiments, the endpoint 102 identifies 8hypotheses for each of the first and second subsets. In someembodiments, at least some of the identified hypotheses of the firstsubset do not match to or otherwise correspond with any of theidentified hypotheses of the second subset.

At block 1008, the endpoint 102 identifies pairs of hypotheses. Eachpair includes a first hypothesis from the first subset and a secondhypothesis from the second subset. In some embodiments, the firsthypothesis is same as the second hypothesis. For example, if the firstsubset includes hypotheses A, B, and C, and the second subset includeshypotheses A, B, and D, then the pairs of hypotheses includes A and B.

In some embodiments, the first hypothesis of the first frame is within apredetermined distance (e.g., range) of the second hypothesis of thesecond frame. That is, a first one or more parameters (e.g., frequency,chip timing, or phase) of the first hypothesis are within one or morepredetermined distances of a second one or more parameters of the secondhypothesis. In some embodiments, the predetermined distance between afirst frequency parameter of the first hypothesis and a second frequencyparameter of the second hypothesis is −100 Hz to 100 Hz. In someembodiments, the difference between the first hypothesis and the secondhypothesis is due to at least one of frequency drift, timing drift, orphase drift. In some embodiments, the endpoint 102 matches (e.g.,matches, links, or maps, groups, includes in the pairs of hypotheses)the first hypothesis to the second hypothesis based on determining thatthe first hypothesis (e.g., the parameters thereof) is within thedistance of the second hypothesis (e.g., the parameters thereof). Forexample, if a first hypothesis of the first subset has a frequency of 1kHz, and the second hypothesis of the second subset has a frequency of1.1 kHz, and all other parameters are the same, and 1.1 kHz is withinthe determined distance of 1 kHz, then the endpoint 102 matches thefirst hypothesis to the second hypothesis (e.g., includes the first andsecond hypotheses in the pairs of hypotheses described above). Inanother example, if the first subset includes hypotheses A, B, C, and D,and the second subset includes hypotheses A′, B′, C′, and E, whereineach of A′, B′, and C′ are within a predetermined range of A, B, and C,respectively, and E is not within a predetermined range of D, then thepairs of hypotheses includes A and A′, B and B′, and C and C′.

In some embodiments, blocks 1002-1008 are performed for each channel atwhich the first frame and the second frame are transmitted by thesatellite 104. In some embodiments, the identified subsets of hypothesesare not the same across the channels for which blocks 1002-1008 areperformed.

At block 1010, the endpoint 102 selects, from the pairs of hypotheses, ahypothesis (e.g., a target hypothesis) based on the second metric. Insome embodiments, the endpoint 102 selects, from the pairs ofhypotheses, a hypothesis having a highest second metric (e.g., orotherwise satisfies a second predetermined or dynamic threshold). Thus,in some embodiments, the selected hypothesis has a higher second metricthan a second metric of any hypothesis in the pairs of hypotheses. Insome embodiments, the hypothesis is selected from the second frame(e.g., from one of the second hypotheses). At block 1012, the endpoint102 selects a channel (e.g., for transmission) at which the hypothesisis selected. For example, the endpoint 102 selects a channel at whichthe hypothesis has the highest second metric. In some embodiments, theendpoint 102 supports further communications with the satellite 104using the selected hypothesis and the selected channel.

Referring now to FIG. 11, an example timing diagram 1100 for Dopplerfrequency offset is shown. The x-axis is time and the y-axis is theDoppler frequency offset. The endpoint 102 includes a processor. Theprocessor has programmed instructions to determine, or have a prioriknowledge of, a satellite 104's altitude (e.g., an altitude at whichit's launched).

The endpoint 102 receives one or more swept syncs from the satellite104. For example, the endpoint 102 receives a first swept sync (e.g., afirst instance of swept sync 304(1) in a first frame) from the satellite104 at time Doppler_(n-1) and a second swept sync (e.g., a secondinstance of swept sync 304(1) in a second frame) from the satellite 104at time Doppler_(n). The processor has programmed instructions tomeasure or otherwise determine one or more frequencies (e.g., Dopplerfrequencies) associated with the one or more swept syncs. The processorhas programmed instructions to determine the Doppler frequency offset(e.g., an initial Doppler frequency offset) and the first derivative ofthe Doppler frequency offset (e.g., an initial first derivative of theDoppler frequency offset) based on the one or more swept syncs (e.g.,based on receiving the swept syncs, based on determining one or morefrequencies associated with the one or more swept syncs). In someembodiments, the Doppler frequency offset is the measured frequency. Insome embodiments, the Doppler frequency is a difference of the measuredfrequency and a predetermined frequency. In some embodiments, theDoppler frequency offset is for a different time (e.g., a time at whichthe endpoint 102 is sending a Tx PDU) to the satellite 104. In someembodiments, the endpoint 102 determines the Doppler frequency offset isfor a different time by estimating (e.g., extrapolating) the frequencyfrom the times Doppler_(n-1) and Doppler_(n) to the different time.

The processor has programmed instructions to calculate (e.g., compute,determine) a second derivative of the Doppler frequency. The secondderivative of the Doppler frequency is calculated based on one or moreof the Doppler frequency offset, the first derivative of the Dopplerfrequency offset, and the altitude of the satellite. In someembodiments, the second derivative of the Doppler frequency iscalculated using a polynomial fit. In some embodiments, the secondderivative of the Doppler frequency is calculated using an orbital modelthat is a function of the Doppler frequency offset and, the firstderivative of the Doppler frequency offset, and an altitude of asatellite 104 in communication with the endpoint 102.

The processor has programmed instructions to calculate (e.g., compute,determine, etc.) a delta for each of the Doppler frequency offset andthe first derivative of the Doppler frequency offset. The deltas arecalculated based on the calculated second derivative of the Dopplerfrequency. In some embodiments, the equations for calculating theDoppler frequency offset delta and the first derivative of the Dopplerfrequency offset delta are:

${{Doppler}\mspace{14mu}{frequaency}\mspace{14mu}{offset}\mspace{14mu}{delta}} = {0.5*{A\left( {\left\lbrack {\frac{T_{F}}{2} + T_{UL}} \right\rbrack^{2} - \left\lbrack \frac{T_{F}}{2} \right\rbrack^{2}} \right)}}$${{First}\mspace{14mu}{derivative}\mspace{14mu}{of}\mspace{14mu}{Doppler}\mspace{14mu}{frequency}\mspace{14mu}{offset}\mspace{14mu}{delta}} = {A\left( {\frac{T_{F}}{2} + T_{UL}} \right)}$

T_(F) is the frame duration between the first swept sync atDoppler_(n-1) and the second swept sync at Doppler_(n). Tut is a timedelta from a time at which the second swept sync at Doppler_(n) wasreceived by the endpoint to the time of a Tx PDU (e.g., an UL PDUequivalent of PDU 306(N), a middle of the Tx PDU, an UL PDU, a middle ofthe UL PDU), of the UL frame is transmitted by the endpoint. In someembodiments, the Tut depends on which slot of the UL the Tx PDU is in(e.g., allocated to).

In some embodiments, the processor has programmed instructions to update(compensate, correct, adjust, predict) the Doppler frequency offset andthe first derivative of the Doppler frequency offset. The processor hasprogrammed instructions to determine (a) the updated Doppler frequencyoffset as a function (e.g., sum, difference, etc.) of the initialDoppler frequency offset and the Doppler frequency offset delta and (b)the updated first derivative of the Doppler frequency offset as afunction of the initial first derivative of the Doppler frequency offsetand the first derivative of the Doppler frequency offset delta. In someembodiments, the updated Doppler frequency offset and the updated firstderivative of the Doppler frequency offset are determined for a timewhen the endpoint 102 sends the Tx PDU to the satellite 104.

Referring now to FIG. 12, an example method 1200 for second derivativecompensation is shown. The method 1200 may be implemented using, orperformed by, an endpoint 102, which is detailed herein with respect toFIG. 1 and FIG. 2. Additional, fewer, or different operations may beperformed in the method 1200 depending on the embodiment.

At block 1202, the endpoint 102 measures a Doppler frequency offset anda first derivative of the Doppler frequency offset. At block 1204, theendpoint 102 calculates a second derivative of the Doppler frequencyoffset. In some embodiments, the endpoint 102 calculates the secondderivative of the Doppler frequency using an orbital model that is afunction of the Doppler frequency offset and, the first derivative ofthe Doppler frequency offset, and an altitude of a satellite 104 incommunication with the endpoint 102. At block 1206, the endpointcalculates a first delta and a second delta to the Doppler frequencyoffset and the first derivative of the Doppler frequency offset,respectively. In some embodiment, the endpoint adds the first delta tothe Doppler frequency offset and the second delta to the firstderivative of the Doppler frequency offset.

In some embodiments, the methods 500-1000 and 1200 may be implemented inone or more processing devices (e.g., a digital processor, an analogprocessor, a digital circuit designed to process information, an analogcircuit designed to process information, a state machine, and/or othermechanisms for electronically processing information). The one or moreprocessing devices may be associated with one or more of the endpoint102, the satellite 104, the ground station 106, or the gateway 108. Theone or more processing devices may include one or more devices executingsome or all of the operations of the methods 500-1000 and 1200 inresponse to instructions stored electronically on an electronic storagemedium. The one or more processing devices may include one or moredevices configured through hardware, firmware, and/or software to bespecifically designed for execution of one or more of the operations ofthe methods 500-1000 and 1200.

It is to be understood that any examples used herein are simply forpurposes of explanation and are not intended to be limiting in any way.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected,” or“operably coupled,” to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable,” to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents and/or wirelessly interactable and/or wirelessly interactingcomponents and/or logically interacting and/or logically interactablecomponents.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to inventions containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to “at least one of A, B, and C, etc.” is used, in generalsuch a construction is intended in the sense one having skill in the artwould understand the convention (e.g., “a system having at least one ofA, B, and C” would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to “at least one of A, B, or C, etc.” is used, ingeneral such a construction is intended in the sense one having skill inthe art would understand the convention (e.g., “a system having at leastone of A, B, or C” would include but not be limited to systems that haveA alone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.” Further, unlessotherwise noted, the use of the words “approximate,” “about,” “around,”“substantially,” etc., mean plus or minus ten percent.

The foregoing description of illustrative embodiments has been presentedfor purposes of illustration and of description. It is not intended tobe exhaustive or limiting with respect to the precise form disclosed,and modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the disclosed embodiments.It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. A method comprising: receiving, by an endpoint, adownlink (DL) PDU from of a plurality of sources; determining, by theendpoint and for each DL PDU, a frequency offset from a referencefrequency; selecting, by the endpoint, one of the plurality of sourceshaving a first frequency offset that satisfies an threshold; andsending, by the endpoint, an uplink (UL) PDU to the selected source. 2.The method of claim 1, further comprising derotating, for each of the DLPDUs, a corresponding swept sync.
 3. The method of claim 1, furthercomprising detecting energy within a predetermined frequency range. 4.The method of claim 3, wherein the predetermined frequency range ispartitioned into bins, and the method further comprises estimating thefrequency offset as a center frequency of a bin having the most detectedenergy of all of the bins.
 5. The method of claim 1, wherein the firstfrequency offset is between −1 kHertz (kHz) and 1 kHz.
 6. The method ofclaim 1, further comprising selecting the one of the plurality ofsources based on the received power.
 7. The method of claim 1, whereinthe DL PDU is included in a DL frame that includes a swept sync.
 8. Themethod of claim 7, wherein the swept sync is used as a proxy for the DLPDU.
 9. The method of claim 1, further comprising applying, to the ULsignal, the frequency offset determined with respect to the selectedsource.
 10. The method of claim 1, further comprising selecting a firstchannel at which to transmit the UL signal, wherein the first channel isbased on a second channel at which the DL frame was received.
 11. Asystem comprising a plurality of sources and an endpoint, wherein theendpoint having programmed instructions that when executed cause anendpoint processor at the end point to: receive a downlink (DL) PDU fromof the plurality of sources; determine, for each DL PDU, a frequencyoffset from a reference frequency; select one of the plurality ofsources having a first frequency offset that satisfies an threshold; andsend an uplink (UL) PDU to the selected source.
 12. The system of claim11, wherein the endpoint processor derotates, for each of the DL PDUs, acorresponding swept sync.
 13. The system of claim 11, wherein theendpoint processor detects energy within a predetermined frequencyrange.
 14. The system of claim 13, wherein the predetermined frequencyrange is partitioned into bins, and the method further comprisesestimating the frequency offset as a center frequency of a bin havingthe most detected energy of all of the bins.
 15. The system of claim 11,wherein the first frequency offset is between −1 kiloHertz (kHz) and 1kHz.
 16. The system of claim 11, wherein the endpoint processor selectsone of the plurality of sources based on the received power.
 17. Thesystem of claim 11, wherein the DL PDU is included in a DL frame thatincludes a swept sync.
 18. The system of claim 17, wherein the sweptsync is used as a proxy for the DL PDU.
 19. The system of claim 11, theendpoint having further programmed instructions to apply, to the ULsignal, the frequency offset determined with respect to the selectedsource.
 20. The system of claim 11, the endpoint having furtherprogrammed instructions to select a first channel at which to transmitthe UL signal, wherein the first channel is based on a second channel atwhich the DL frame was received.